1. Field of the Invention
The present invention is directed to a method for generating an image from magnetic resonance signals of the type wherein a first image matrix is generated from first magnetic resonance signals and a second image matrix is generated from second magnetic resonance signals, an overall magnitude is formed for each picture element by adding or subtracting a magnitude dependent on the appertaining magnitude value of the second image matrix to or from a magnitude that is dependent on the magnitude value of the corresponding picture element of the first image matrix, with the overall magnitudes being employed for generating the image.
The present invention also is directed to a method for generating an image from magnetic resonance signals of the type wherein a first group of location coded magnetic resonance signals and a second group of location coded magnetic resonance signals different therefrom are received, overall signals are formed from the respective magnetic resonance signals with coinciding location coding of the first group and the second group, wherein the overall signal is formed by adding or subtracting a second magnitude dependent on the appertaining magnetic resonance signals of the second group to or from a first magnitude that is dependent on the magnetic resonance signals of the first group, and wherein the overall signals are utilized for reconstruction of the image.
The invention also is directed to a magnetic resonance tomography apparatus for the implementation of the method and to a data processing system for a magnetic resonance tomography apparatus.
2. Description of the Prior Art
It is known in the field of magnetic resonance tomography to add or subtract two congruent images generated in different ways, i.e. images that constitute the same portion in the examination subject. The magnitude value of the appertaining picture element of the second image is added to or subtracted from a magnitude value of a picture element of the first image for each picture element. In this way, contrasts can be intensified, new contrasts can be generated or image artifacts can be avoided. The congruent images are generated from a common pulse sequence, i.e. they derive from a readout sequence with common phase coding.
The picture elements are represented in the images as complex numbers, and the magnitude value of a complex number a+bi is (a2+b2)1/2 (also called the absolute value).
An image addition is implemented, for example, in the DESS technique (Double Echo Steady State). A specific pulse sequence is generated with which a FISP echo as well as a PSIF echo can be read out within a readout train. The DESS method is disclosed, for example, in the article by W. Nitz in electromedica 65 (1997), No. 1. The FISP echo (Fast Imaging with Steady State Precession) is a gradient echo. In the preferred field of application of orthopedics, it supplies a T1/T2 contrast typical of steady state techniques (SSFP pulse sequences). The PSIF echo arises from a FSIP pulse sequence that sequences backwards. This is also referred to as a quasi-spin echo. Dependent on the repetition time, it carries a strong T2 contrast. The magnitude addition of the image resulting from the FISP echo with the image resulting from the PSIF echo supplies an image with good anatomy and very good emphasis of fluid, for example the synovil fluid, at pathological locations.
An image subtraction is known, for example, from German OS 196 16 387. The HIRE method (high-intensity reduction sequence) is disclosed therein. After an excitation, two groups of magnetic resonance signals are acquired in two time spans at a different intervals from the excitation. An image is acquired on the basis of the signal differences of respective magnetic resonance signals of the first and second groups with coinciding location coding. The first group of magnetic resonance signals or echos, which is acquired shortly after the excitation, results in an image with normal T2 weighting. The second group of magnetic resonance signals or echos, which is acquired later in a time span than the first group and wherein a tissue part having a longer T2 time constant supplies the significant signal contribution, yields a highly T2-weighted image. A fluid such as, for example, the cerebral spinal fluid (CSF), leads to a very high signal contribution in a normally T2-weighted image, for example to a significantly higher signal contribution in the brain than the other brain areas. In the normally T2-weighted image acquired shortly after the excitation, a neighboring image region would be over-shadowed by this high signal contribution of the CSF and the resolution would thus be locally diminished. Moreover, artifacts referred to as CSF flux or pulsation artifacts also arise. When the highly T2-weighted image acquired at a later point in time is subtracted from the magnitude image acquired shortly after excitation, then an image results that is still T2-weighted and wherein the fluid, particularly CSF, is highly suppressed.
One disadvantage of the known image subtraction or image addition methods is that the aggregate noise increases approximately by a factor of √{square root over (2)}.